On Orbit Performance of the HST Optical Control System

The function of the Optical Control System (OCS) was to align the Optical Telescope Assembly (OTA) to achieve a λ/20 on-axis wavefront Its existence was dictated by the error budgets for aligning the Secondary Mirror (SM) to the Primary (PM) on the ground and for stability of the PM figure, and by the need to accomodate moisture desorption of the Metering Truss Assembly (MTA). The alignment system needed to accomodate the large OPD error which could, under worst case conditions, have been present immediately after launch, while also achieving measurement accuracies of λ/80 in the aligned state. This was to be accomplished by using a Fine Guidance Sensor (FGS) for coarse alignment, and an interferometric Wavefront Sensor (WFS) for fine alignment, both working in conjunction with the Actuator Control System (ACS). Since the final OTA wavefront error requirement was with reference to a perfect surface, an absolute reference was needed on-orbit, or equivalently, the system had to be calibrated on the ground, and thereafter had to remain stable for the life of the mission. To the extent that can be determined from a variety of on-orbit measurements, the OCS hardware and software is capable of these exacting requirements. However, the OCS does not measure to the accuracy required, apparently because spherical aberration in the PM has rendered the coma component of the ground calibration invalid.